3. How does light affect living organisms?

3.1 What is light and how is it absorbed and measured?

The SCENIHR opinion states:

3.4. First principles and biology

3.4.1. Optical radiation

Wavelengths of visible EM radiation range from 400 to 780 nm
(1 nm = 10-9 m), spanning the visible range from violet to red
light (see CEI/IEC 2006/62471, Directive 2006/25/EC2). In
article 2a of Directive 2006/25/EC the visible range is
positioned more broadly between 380 and 780 nm. Light can be
manipulated by a variety of optical devices or elements; most
characteristically a beam of light can be focused or diverged by
optical lenses made of crystal (quartz) or glass, as in
binoculars, telescopes and cameras. Optical radiation
encompasses light but also includes EM radiation of wavelengths
well beyond the visible range: ultraviolet (UV) radiation is
below 400 nm down to 100 nm and
infrared (IR)
radiation is above 780 nm up to 1 mm. UV and IR radiation can
also be manipulated by optical devices and elements such as
optical lenses (sometimes optical radiation is referred to as
“light”, and one then speaks of “UV light” and “IR light”, next
to “visible light”; here
the latter is considered a tautology and the former two are
consequently oxy).

The UV band is sub-divided in three wavelength regions (CIE
2006/62471):

UVA from 400–315 nm

UVB from 315–280 nm

UVC from 280–100 nm

The IR-band is similarly sub-divided in three wavelength regions (CIE 2006/62471):

IRA from 0.78 to 1.4 μm (μm = 10-6 m)

IRB from 1.4 to 3.0 μm

IRC from 3.0 μm to 1mm

Formally, this leaves the stricter range of 400-780 nm as the
wavelength range of visible
radiation, light.

Although the sun emits optical radiation over the full
wavelength range, the earth’s atmosphere blocks UVC and part of
the UVB irradiation below 290-295 nm (mainly by oxygen and
stratospheric ozone) and IRC of wavelengths over 30 μm (by water
vapour). Interestingly, the sun’s spectrum peaks over the
visible range. Although UV is classified as non-ionizing
radiation, it can cause chemical reactions, and causes many
substances to fluoresce. Most people are aware of the effects of
UV irradiation through the painful condition of sunburn, but UV
irradiation has many other effects, both beneficial and
damaging, to human health.

For optical radiation to have an effect on matter the
radiation needs to be absorbed, i.e. the radiant energy needs to
be transferred to the material in which the effect is to occur.
Two main mechanisms can be distinguished through which the
absorbed radiant energy can take effect:

a) Heat: radiant energy is converted into molecular motion
(kinetic energy) such as vibration, rotation and translation.
Thus the temperature is increased (photothermal effect). Here,
the radiant energy (measured in Joules, J) absorbed per unit
time (s) in a certain volume determines the rise in temperature,
i.e. the absorbed radiant power (J/s = Watt, W) per unit volume
(m3) or the (specific) absorption rate
(W/m3) is the determining factor (next to how fast
the absorbing volume is cooled by heat exchange with its
environment).

b) Photochemistry: radiant energy can cause excitation of
atoms or molecules by moving the outermost (valence)
electrons to
higher orbital energy levels. This energy can subsequently be
utilized in (photo-)chemical reactions, yielding
“photoproducts”. The radiation needs to be within a certain
wavelength range (the “absorption band”) for the excitation to
take place as the radiant energy is absorbed in discrete quanta,
“photons”, which must match the energy required for the
excitation. The (part of the) molecule that absorbs the
radiation is dubbed the chromophore. Not every excited molecule
will cause a chemical reaction: the energy may be lost through
fluorescence (emission of radiation of longer wavelengths) or
dissipated as heat. This implies that only a certain fraction of
the absorbed radiant energy is channelled into the (photo-)
chemical reaction: this is represented by the quantum efficiency
(the number of photoproducts formed per photon absorbed; a ratio
usually <1). The absorbing molecule is not necessarily
the molecule that is chemically altered; the energy can be
transferred to another molecule, which may then become
chemically reactive (e.g. radicals and reactive oxygen species
may thus be formed). In general, the total radiant energy
(radiant power times exposure time in W x s=J) absorbed by the
proper chromophores determines to what extent the
photochemical
reaction has evolved, i.e. the amount of photoproduct
formed.

Of the three types of optical radiation, UV radiation is
photochemically most active (the photons carry the highest
energy), and it is absorbed by certain common chromophores in
organic molecules (e.g. C=O, C=S and aromatic rings; the latter
are abundantly present in DNA (Figure 3)). Clearly, light is also
photochemically active in the eye: visual perception starts with
the photo-isomerisation of opsin proteins (in G-protein coupled
receptors which trigger the neural signalling). In the skin
there are also other chromophores that absorb light. For
example, heme-ring structures are present in enzymes, such as
cytochrome-c oxidase in the mitochondria. This enzyme is even
sensitive to IRA radiation of wavelengths around 820 nm (Karu et
al. 2004) by excitation of a copper atom. However, by and large,
IR radiation is not capable of moving valence
electrons to
higher energy levels (the energy transferred per photon is too
low for excitation of valence electrons) and thus initiate
photochemical
reactions. Most IR effects are heat-mediated.

The light interacts with eye tissues and molecules through
different mechanisms. Some of the eye tissues or pigments can
absorb light and thus reduce retinal exposure. In other parts of
the eye or pigment structures, the light can induce oxidative
stress damage defined as
photochemical and
photodynamic effects.

Figures 4 a-d below show the penetration/absorption of
radiation by the eye for different age groups (all figures
adapted from Sliney 2002).

Figure 5. Light penetration in the skin
(attenuation down to 1% occurs for light wavelengths of 250-280 nm
at around 40 μm depth; for 300 nm at 100 μm; for 360 nm at 190 μm;
for 400 nm at 250 μm; for 700 nm at 400 μm; for 1.2 μm at 800 μm;
for 2 μm at 400 μm; for 2.5 μm at 1μ; and for 400 μm at 30 μm)

3.4.2.2. Photobiology and dosimetry

In photobiology, optical radiation usually penetrates a body
through the outer surface (skin or eye), and the exposure
(radiant energy per surface area in J/m2) and
exposure rate or irradiance (radiant energy per surface area per
unit time in J/m2s, W/m2) are the commonly
used proper photobiologic metrics by which to quantify the
transfer of radiant energy to the body. However, by convention,
in some disciplines such as ophthalmology and dermatology the
exposure is most often given as mJ/cm2. This
convention is also followed in this document. The eye has the
special feature of focusing the light onto the
retina whereby the
irradiance from the surface of the eye to the retina is
increased by several orders of magnitude (up to 200,000-fold;
see University of Waterloo safety office).
The irradiance at the retina over the image of a light source
(either a lamp or an object reflecting light) is determined by
the diameter of the pupil and the radiance of the light source.
The radiance is the power transmitted into a solid angle onto
the pupil per surface area of the source (in
W/sr.m2). Interestingly, the distance from the light
source drops out of the equation for a source with a homogeneous
radiance over its surface (see Box I) if the light is not attenuated by
absorption or scattering in the air between the eye and the
light source. At greater distances the pupil catches less of the
light from the source, but as the image of the source becomes
smaller with larger distances, more of the radiant surface is
projected onto a small area on the retina. These loss and gain
with distance cancel each other out, leaving the irradiance in
the image area on the retina unchanged. It should be noted that
a very bright source will cause immediate aversion and thus will
not be focused on for any substantial length of time. The skin
remits by back scatter much of the incoming visible and IRA
radiation but absorbs most of the UV and IRB and IRC radiation.
The penetration of the optical radiation into the tissue (skin
or eye) determines to what depth effects or damage can occur,
but also over which volume of tissue the absorbed radiant energy
is spread; Figures 4 (a-d) and 5 illustrate the penetration of
UV, visible and IR radiation (only depicted for skin) into the
eye and the skin, respectively. From these figures it is clear
that visible and IRA radiation penetrate deepest into the skin
(10- fold reduction at 0.1-0.4 mm depth) and eye (onto the
retina), whereas UVA and UVB radiation reach the
lens in the eye.
Short wavelength UVC and long wavelength IRB and IRC penetrate
the skin only very shallowly and do not reach the lens in the
eye. The superficial absorption of broad-band IRB and IRC
radiation implies that most of the radiant energy is absorbed in
a very thin layer which can consequently be heated efficiently.

In the IRB and IRC region of the spectrum, the ocular media is
opaque as a result of the strong absorption by their constituent
water. Beyond a wavelength of 1.9 μm the
cornea becomes the only
absorber. Direct exposure to high levels of IRC
(>1W/cm2) may induce corneal lesions,
particularly of the epithelium. The human cornea transmits
radiant energy only at 295 nm and above (and thus not in the UVC
range). Indeed, all UVC (100-280 nm) radiations are absorbed by
the human cornea which absorbs radiation. It absorbs light very
efficiently, over 90%, between 300- 320 nm (UVB range), about
30-40% between 320-360 nm (UVA range) and almost 100% above 800
nm (i.e. IRA, IRB and IRC ranges) (Sliney 2002). Almost no
absorption occurs in the spectrum of
visible radiation.

However, the part of UVA that is transmitted from the
cornea is absorbed
in the aqueous humour, the
lens and even in the
vitreous. Indeed, about 45-50% of the UVA is absorbed by the
lens. Part of the UVA transmitted by the lens is then absorbed
by the vitreous, so that only 1-2% of the UVA reaches the
retina. In young
children (at about or just below 9 years of age, where the limit
is approximate since no study has clearly defined it), a window
exists that allows transmission of about 2-5% of UV at 320 nm to
the retina (Gaillard et al. 2000). At older ages no UV at this
wavelength reaches the retina (Dillon et al. 2004). The other
main difference in young children compared to adults and older
children is the transmission of blue light by the lens. Around
15% of 400 nm and about 65% of 460-480 nm wavelengths reach the
retina in children less than 9 years of age, compared to 60% at
460-480 nm at 10 years. In the age group of 60-70 years, only
ca. 1% at 400 nm and 40% at 460-480 nm reaches the retina. This
difference is explained by the fact that the colour of the lens
becomes more and more yellow with increasing age (Gaillard et
al. 2000). It is important to note that even without any
clinically detectable
cataract, changes in
transmission are occurring in the lens (Ham et al. 1978). The
age at which transmission of blue light decreases may be
variable due to genetic, nutritional and exposure factors.
Therefore the percentages given in these schemes are approximate
and intended to give a range of ocular media transmission as a
function of age and spectrum.

Although some photochemically mediated biological effects may
depend on the total amount of photoproducts irrespective of the
spatial distribution, others may depend on the density of
photoproducts, i.e. the amount per surface area or volume. If
the photoproducts are removed from the tissue (dead
cells in days) or
repaired (DNA damage in hours to days), the effect in the tissue
will evidently depend on how quickly the photoproducts are
generated. After absorbing light, visual pigments (opsins) take
minutes to get regenerated (Sandberg et al. 1999). Following
exposure of the eye to very intense illumination, a greatly
elevated visual threshold is experienced, which requires tens of
minutes to return completely back to normal. The slowness of
this phenomenon of “dark adaptation” has been studied for many
decades, yet is still not fully understood. Upon photon
excitation, rhodopsin undergoes photoactivation and bleaches to
opsin and all-trans-retinal. To regenerate rhodopsin and
maintain normal visual sensitivity, the all-trans isomer must be
metabolized and reisomerized to produce the chromophore
11-cis-retinal. This constitutes the visual cycle, which
involves the retinal pigment epithelium, where all-trans
retinoid is isomerized to 11-cis-retinol. The time- course of
human dark adaptation and pigment regeneration is determined by
the local concentration of 11-cis retinal. After intense light
exposure, the recovery is limited by the rate at which 11-cis
retinal is delivered to opsin in the bleached rod outer
segments.

Radiations of different wavelengths will generally differ in
the efficiency by which they trigger a chemical reaction or
evoke a biological response; i.e. the wavelength at which a
smaller exposure is required for a certain (level of) response
is more efficient (such differences largely depend on the
absorption spectrum of the relevant chromophore and the
transmission of the radiation through the medium or tissue to
the chromophore). The wavelength dependence of this efficiency
is dubbed an “action spectrum” (a wavelength by wavelength plot
of the inverse of the exposure needed for a certain response).
Such an action spectrum can be used for spectral weighting of
the exposure to a source to ascertain the biologically effective
exposure or photobiologic dose (for details in formulae see Box
I).

The European Standard EN 62471 recommends evaluating the
Photobiological Risk Group for General Lighting Systems (GLS) at
a distance where the horizontal illuminance is 500 lx. However,
the same standard underlines that for all other lamp types this
evaluation has to be carried out at 200 mm. The two
recommendations are consistent with two distinct risks: the
first (500 lx) corresponds to the situation for a worker in a
well- illuminated environment without direct view of the light
source; the second (200 mm) is more appropriate for evaluating
the risk of a person looking directly in the direction of the
light source. Following this reasoning, it is recommended to
evaluate the risk class based on the potential use of the light
source by the end-user. For example, light sources within
ceiling fixtures or indirect lighting can be characterized at
500 lx level, whereas task lights, downlights, etc. that can be
in the line of sight should be evaluated at 200mm.

3.2 How can light affect biological systems?

The SCENIHR opinion states:

3.4.3. Biological effects

Overexposure can cause dysfunction or outright destruction of
tissue, either through heating or
photochemical
reactions. As implied by the term “overexposure”, a
certain threshold of tolerable levels of exposure or irradiance
is surpassed: the irradiance can become too high and cause
thermal damage or the accumulated exposure carries a
photochemical
reaction to a toxic level. It should be stressed that
this does not imply that there is no biological effect below the
threshold level, but the damage is minor and tolerable
(non-destructive) and/or the absorbed radiant energy causes a
functional biological response (receptive absorption). Below we
present biological effects from “receptive absorption” or by
“destructive or toxic overexposure”.

3.4.3.1. Photothermal effects

A. Reception

Absorption of optical radiation by the skin will cause heating
which can raise the temperature. The skin can sense temperature
differences smaller than 0.1°C on the face, especially on the
lips (Jones 2009, Stevens and Choo 1998). The skin is innervated
by axons (nerve endings from neurons residing in the spine)
which carry transient receptor potential (TRP) ion channels that
are sensitive to temperature changes in their cell membranes.
Some axons carry TRP channels that are activated below certain
temperatures, sensing cold, whereas others are activated above
certain temperatures thus giving a hot sensation (some of these
TRPs are also present on the tongue and respond to menthol, a
“cool” sensation, and capsaicin in pepper, a “hot” sensation
(Denda et al. 2010). Very recently, transient receptor potential
vallinoid (TRPV) channels have also been found in human
corneacells (Mergler et al.
2010). They may be involved not only in thermo-sensation, but
also in the regulation of cell proliferation. In the
retina, TRPV
channels have been identified that are more sensitive to
pressure than temperature (Sappington et al. 2009).

B. Damage

Proteins can become denatured (loss of tertiary structure) at
high temperatures and,
cells and tissue
irreversibly damaged in 15 to 60 minutes at 45°C (Kampinga et
al. 1995) and in a matter of seconds at 60-70°C (Biris et al.
2009, Priebe et al. 1975). Pain and retraction reflexes
evidently serve to limit the damage. Blisters may develop first
due to loss of adherence between skin layers. Limited
superficial thermal wounds, as from cosmetic or therapeutic skin
ablation by laser treatment, can be restored from deeper and
neighbouring layers of skin, but extensive third degree deep
burns need special medical care and skin transplants. The
immune system will
respond to thermal damage by an
inflammatory reaction
in the skin. Regarding thermal damage to the eye, only pulsed
lamps are of concern. If the rate of energy deposition is faster
than the rate of thermal diffusion (thermal confinement), then
the temperature of the exposed tissue rises. If a critical
temperature is reached (typically about 10°C above basal
temperature), then thermal damage occurs. Thermal injury is
caused mainly by absorption of light wavelengths >450 nm
by the retinal pigment epithelium; the effects are usually
immediate. Thermal burn is rare unless the light source is
pulsed or in near contact with the eye. Thermal damage usually
does not occur with domestic lights but can be induced by pulsed
lamps and lasers. In such cases, retinal damage is primarily
induced via thermal mechanisms for exposures shorter than 5
seconds. During longer exposure times both thermal and
photochemical
damage takes place.

The iris responds to light by constriction, the pupillary
reflex, thus reducing light entry into the eye. This mechanism
is extremely important and efficient for protecting the
retina against
light damage. Pupillary constriction is highly dependent on the
wavelength. Lucas et al. (2001) showed that the pupillary light
reflex in mice was driven by a non- rod, non-cone photoreceptive
system using a photopigment with peak sensitivity around 479 nm
(melanopsin). The work of Hattar et al. (2003) recognized the
melanopsin- associated photoreceptive system as being
responsible for conveying photic information for accessory
visual functions such as pupillary light reflex and circadian
photo- entrainment. In humans, light pupillary constriction is
achieved at a peak sensitivity of 482 nm and the sustained,
post-stimulus pupil constriction is mediated predominantly by
the melanopsin-driven, intrinsic photoresponse and not by
sustained rod activity resulting from bleached rhodopsin as had
previously been suggested. Light pupillary constriction is
observed at 5 cd/m2 at 482 nm in humans and primates
(Gamlin et al. 2007). The retina The peak of absorption of the
retina is between 400 and 600 nm and its transmission is between
400 and 1,200 nm. Rods are present across the retina except for
the very central region (the foveola), and provide scotopic
(night) vision. Their sensitivity is 10-6- 1 cd/m2,
with comparatively low resolution and high sensitivity, but
lacking colour information. Their absorption peak is at 498 nm
(blue), but in vivo, if the
lens absorption and macular
pigments are taken into account, the effective maximum
sensitivity of the rod integrated in the eye is shifted to 507
nm. Cones are responsible for daylight (photopic) vision. Their
sensitivity varies in a wide luminance range, from 10-3-108
cd/m2. Maximal absorbance for blue cones is
around 450 nm, 530 nm for green cones and 580 nm for red cones.

The visual pigment in the rod is rhodopsin, which consists of
opsin and the vitamin A
aldehyde 11-cis-retinal. Phototransduction is triggered by the
photic conversion of 11- cis-retinal to all-trans-retinal in the
rhodopsin molecule. The activation of rhodopsin starts a cascade
of events that leads to the closure of sodium channels,
hyperpolarization of the photoreceptor membrane, and a decrease
in the concentration of intracellular calcium (Pepe 1999). The
phototransduction system can be modulated by several proteins
(such as S-modulin [recoverin], S-antigen [arrestin], guanylate
cyclase- activating protein, phosducin, and calmodulin) in a
calcium-dependent manner, inducing light and dark adaptation.
Rhodopsin is regenerated in the retinal pigment epithelial (RPE)
cells through the
visual cycle of retinoid metabolism (Bok 1990, Saari et al.
1994).

B. Damage B1. In the eye The
cornea Exposure of the
cornea to UVA and UVB usually induces reversible lesions of the
corneal epithelium. UVC can induce lesions of the corneal stroma
and the Bowman membrane leading to corneal opacity and
potentially to corneal neovascularization. IR usually only
causes irritation but may, at high energy levels (>3
mJ/cm2), also cause deep stromal lesions and even
perforations. Protection from IR and UV components of the
sunlight is therefore recommended in certain instances (Sliney
2001). Upon prolonged exposure to UV (sunlight), climatic
droplet keratopathy and cortical
cataracts
(opacification of the cortex of the
lens and not the
nucleus) can occur.
On the conjunctiva,
pterygium and conjunctival neoplasms can be observed. Ocular
melanoma (mostly
uveal melanoma) might also be induced by UV overexposure.
Evidence for an association between ocular melanoma and sun
exposure comes from Australia. A national case-control study of
ocular melanoma cases diagnosed between 1996 and mid-1998
demonstrated an increase in risk of the
cancer with increasing
quartile of sun exposure prior to age 40 (relative risk (RR) in
the highest quartile 1.8; 95% CI 1.1–2.8), after control for
phenotypic susceptibility factors (Vajdic et al. 2002). The
subclinical photokeratitis level normalized to the UV-hazard
action spectrum peak at 270 nm wavelength is approximately 4
mJ/cm2 (as defined by ACGIH and ICNIRP, and
stated in EU Directive 2006/25/EC). The radiant exposure at 300
nm that would be equivalent to the corneal exposure of 4
mJ/cm2 at 270 nm is 10 mJ/cm2. Between
315- 400 nm, the exposure guideline limit is 1 J/cm2
for t <1000 s. The lens The lens absorbs near UV and far
infrared light
(<400 and >800nm) (Boettner and Wolter 1962). It
is known that UV light induces cataracts (Hockwin et al. 1999,
Sasaki et al. 1999) with a damage threshold of 600
mJ/cm2 at 350 nm.

A corresponding value for 310 nm is 750 mJ/cm2.
Blue light may induce photodynamic damage in lenses which have
accumulated photosensitive debris or drugs. Other compounds that
accumulate in the aging
lens may act as
antioxidants (Balasubramanian 2000).
Infrared may also
cause cataracts (Roh and
Weiter 1994). Cortical cataracts have been associated with UV
exposure. Furthermore, it seems that exposure to UV at younger
ages also predisposes individuals to nuclear cataracts later in
life (Neale et al. 2003).

Whether cumulative UV exposure from artificial lighting, added
to the natural light exposure, might increase the incidence of
cataract at younger
ages was never investigated. The absorption spectrum of the
lens changes with
age. In young children, more than 80% of blue light is
transmitted to the retina.
At around 25 years of age, only 20% of the light between 300 and
400 nm and 50% of wavelengths between 400 and 500 nm is
transmitted. With increasing age, the yellow filters of the lens
increase and absorb most of the blue light. The peak of
absorption of the lens is around 365 nm in young adults and
around 400 nm at 60 years. This natural retinal protection of
the lens, increasing with age, tends to be replaced in the case
of cataract surgery by yellow intraocular lenses (Margrain et
al. 2004).

Light (particularly short wavelengths) can interact with
photoreceptor associated opsins and retinoids and cause damage
via the overproduction of reactive oxygen species (ROS) (Boulton
et al. 2001), but such damage can also arise outside the
photoreceptors. In the
retina,
photochemical damage
through oxidative stress takes place when the incident radiation
has a wavelength in the high energy portion of the visible
spectrum. The retina, which contains a large concentration of
cell membranes, is particularly sensitive to oxidative stress
because lipid peroxidation breaks down membranous structures.
The photochemical damage spreads from the absorbing molecule to
other molecules in an uncontrolled molecular chain reaction.
There are two classes of photo- damage (see also overview in
Table 2 below):

Class I damage has an action spectrum that is identical to
the absorption spectrum of the visual pigment, and it appears
after exposure (of several hours to weeks) to irradiances below
10 W/m2 of white light estimated at the retina. For
comparison, an approximation calculated for this document
suggests that the retinal illuminance caused by the sun shining
on snow or white sand on a clear day is in the order of 30- 60
W/m2. The initial damage is mainly located in the
photoreceptors, where reactive oxygen species (ROS production)
can be measured upon blue light exposure in vitro (Figure
7). However, depending on the
species, it seems that both RPE and photoreceptor cells can be
the primary target sites.

Class II damage has an action spectrum that peaks at
shorter wavelengths, and this type of damage occurs following
exposure to high irradiances of white light, at or above 100
W/m2. The initial damage is generally confined to
the retinal pigment epithelium (lipofuscin-mediated) but may
then extend to the photoreceptors. In RPE cells, lipofuscin
granules are converted to melanolipofuscin in aging eyes, and
the lipofuscin becomes much more phototoxic and particularly
sensitive to blue light with increasing age, meaning that more
free radicals may be produced in the eyes of elderly people. The
damage that occurs in RPE cells and subsequently in
photoreceptor cells is irreversible. The damage occurrence
depends on the anti- oxidant status of the retina and also on
the local oxygen tension in the outer retina. The photooxidative
damage taking place in the outer retina is
cumulative.

Other pigments exist in mitochondria in all tissues but are
particularly susceptible to
photochemical damage in
ganglion cells that receive
light directly on the retinal surface. Their peak absorption is
also in the blue spectrum (e.g. peak at 450 nm for
flavine).

Macular pigments

In the macula of the
retina, yellow pigments
located in the inner retinal layers are particularly
concentrated in the fovea. The lutein and zeaxanthin pigments
efficiently absorb blue light between 400 and 500 nm (Whitehead
et al. 2006). Lutein protects against oxidative damage and is a
scavenger for singlet oxygen (Davies and Morland 2004, Krinsky
et al. 2003, Li et al. 2010, Wooten and Hammond 2002). However,
humans cannot synthesize macular pigments. They are highly
concentrated in the macula of children and additional amounts of
macular pigment can only be achieved through nutrient intake.
Nutrient supplements have been shown to increase macular pigment
density in older patients and are therefore considered to reduce
the risk for progression of age-related macular degeneration
(AMD) (Carpentier et al. 2009, Loane et al. 2008).

Lipofuscin

RPE cells are polarized
epithelial cells with long microvilli on their apical surfaces,
interfacing with the outer segments of photoreceptor cells. The
tight junctions between RPE cells constitute the outer
blood-retinal-barrier, selectively controlling the passage of
water and ions between the subretinal space and the choroids.
RPE cells play a crucial role in the phagocytosis of
photoreceptor outer segments and regeneration of visual pigments
(Bok 1990). At their apical side, RPE cells contain
intracellular melanin
granules (eumelanin and pheomelanin) as well as many
microperoxisomes and antioxidative enzymes, which act as
protective and anti-oxidative mechanisms. Particularly, melanin
absorbs the excess of photons from 300 to 700 nm. Lipofuscin is
a mixture of chromophores that accumulates in the retinal
pigment epithelium with age and in the case of several retinal
disorders. It is a potent photosensitizer capable of inducing
photodynamic effects and subsequent
photochemical processes
(Boulton et al. 1990, Wang et al. 2006), possibly causing
permanent damage to RPE and photoreceptors (Wassel et al. 1999).
The major fluorescent component of lipofuscin, A2E, has been
identified (Sakai et al. 1996). A2E is formed in rod outer
segments by a sequence of reactions that is initiated by the
condensation of two molecules of all-trans-retinaldehyde with
phosphatidylethanolamine. It has a visible absorption maximum
between 430 and 440 nm, depending on the solvent, and generates
light induced ROS (Parish et al. 1998, Reszka et al. 1995).
Interestingly, age- induced changes in the lipofuscin
composition and structure increase its photodynamic effect upon
illumination, resulting in higher oxidative damage (Wu et al.
2010).

As pointed out earlier, UV radiation is very (photo-)
chemically active on a large variety of organic molecules, most
prominently on DNA. Next to direct damage to molecules like DNA,
UV radiation can generate reactive oxygen species and various
kinds of radicals which can then damage cell components. At low
UV exposure levels the skin is perfectly capable of coping with
this UV challenge through antioxidants, radical scavengers and
repair mechanisms, and the exposure will have no direct
noticeable effect (see de Gruijl 1997). If the exposure, and the
damage, increases to levels where the functions of the cell
become seriously disturbed, the cell may become apoptotic
(undergoing programmed cell death). The UV radiation at higher
levels has a clear toxic impact which evokes an
inflammatory
reaction. In the long term, sub-acute damage may cause
accumulation of gene mutations in the (stem)
cells of the
epidermis (causing
cancer) or cause
loss of collagen in the
dermis with a subsequent gradual loss of elasticity
(“photo-aging”). Specific UV signature mutations (at sites of
neighbouring pyrimidine bases in the DNA) were found in p53
tumour suppressor genes in a majority of human skin carcinomas,
providing direct evidence that UV radiation had contributed to
the development of these tumors (de Gruijl et al. 2001).